In short:
Wikipedia mentions a required "availability search" to find peers (and the actual file):
Note that, although a particular file is indicated, an availability search for it must still be carried out by the client application.
Where does the client look? Does a magnet link require a tracker URI or is that up to the client's network?
More info:
A certain magnet URI/URN from tpb looks like this:
magnet:?xt=urn:btih:e9b785fc2d70811a72df5a76bb34bd2eaf9df956&dn=Dances+with+Wolves+1990+20th+Anniversary+Extended+Cut+720p+BRRip&tr=udp%3A%2F%2Ftracker.openbittorrent.com%3A80&tr=udp%3A%2F%2Ftracker.publicbt.com%3A80&tr=udp%3A%2F%2Ftracker.istole.it%3A6969&tr=udp%3A%2F%2Ftracker.ccc.de%3A80
It contains 4 tr
query params with (I suppose) tracker locations that contain some sort of hash index. However, Wikipedia doesn't mention the tr
param, so I assume it's not mandatory.
Where does a client start looking for the file if no tracker URI's are included? And if there are? I can imagine a torrent client (like uTorrent) itself having an enormous index of file hashes.
tr
parameter (at least at the time I was viewing it's article)